OTLP metrics sender interface (#5691)

* Add capability for anyone to define their implementation of OTLP metrics sending

* Fix formatting

* Remove otlp proto from API and introduce Builder

Keeps the API more generic without a dependency on API from the opentelemetry-proto-java project. Also adds a Builder to avoid the need for more public constructors that may change in the future.

* Polish

* Add docs

---------

Co-authored-by: Tommy Ludwig <8924140+shakuzen@users.noreply.github.com>

docs/build.gradle

@@ -47,9 +47,12 @@ dependencies {
 
     testImplementation project(':micrometer-test')
     testImplementation project(':micrometer-observation-test')
+    testImplementation project(':micrometer-registry-otlp')
     testImplementation libs.aspectjweaver
     testImplementation libs.junitJupiter
     testImplementation 'org.assertj:assertj-core'
+    // needed for OtlpMeterRegistryCustomizationTest
+    testRuntimeOnly libs.okhttp
     testImplementation(libs.spring6.context)
     testImplementation 'io.projectreactor:reactor-core'
     testImplementation 'io.projectreactor:reactor-test'

docs/modules/ROOT/pages/implementations/otlp.adoc

@@ -66,6 +66,27 @@ management:
 
 If this config is empty, resource attributes are loaded from the `OTEL_RESOURCE_ATTRIBUTES` environmental variable. You can override `service.name` by setting the `OTEL_SERVICE_NAME` environmental variable, and this takes precedence over other configs.
 
+== Customize metrics sender
+
+The `OtlpMeterRegistry` has an `OtlpMetricsSender` abstraction for sending batches of metrics in OTLP protobuf format.
+
+By default, metrics from the OTLP registry are sent via HTTP to the URL specified by `OtlpConfig#url` using an `HttpUrlConnectionSender`.
+You may use a different `HttpSender` implementation by creating and configuring an instance of `OtlpHttpMetricsSender` such as in the following example using the `OkHttpSender` instead of the default `HttpUrlConnectionSender`:
+
+[source,java,subs=+attributes]
+-----
+include::{include-java}/metrics/OtlpMeterRegistryCustomizationTest.java[tags=customizeHttpSender, indent=0]
+-----
+
+You can also provide a custom implementation of `OtlpMetricsSender` that does not use HTTP at all.
+For instance, if you made a gRPC implementation, you could configure it in the following way.
+Micrometer does not currently provide a gRPC implementation of `OtlpMetricsSender`.
+
+[source,java,subs=+attributes]
+-----
+include::{include-java}/metrics/OtlpMeterRegistryCustomizationTest.java[tags=customGrpcSender, indent=0]
+-----
+
 == Supported metrics
 https://opentelemetry.io/docs/specs/otel/metrics/data-model/#metric-points[Metric points, window=_blank] define the different data points that are supported in OTLP. Micrometer supports exporting the below data points in OTLP format,
 

docs/src/test/java/io/micrometer/docs/metrics/OtlpMeterRegistryCustomizationTest.java

@@ -0,0 +1,57 @@
+/*
+ * Copyright 2025 VMware, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.micrometer.docs.metrics;
+
+import io.micrometer.common.lang.NonNullApi;
+import io.micrometer.core.ipc.http.OkHttpSender;
+import io.micrometer.registry.otlp.OtlpConfig;
+import io.micrometer.registry.otlp.OtlpHttpMetricsSender;
+import io.micrometer.registry.otlp.OtlpMeterRegistry;
+import io.micrometer.registry.otlp.OtlpMetricsSender;
+import org.junit.jupiter.api.Test;
+
+import java.util.Map;
+
+class OtlpMeterRegistryCustomizationTest {
+
+    @Test
+    void customizeHttpSender() {
+        // tag::customizeHttpSender[]
+        OtlpConfig config = OtlpConfig.DEFAULT;
+        OtlpHttpMetricsSender httpMetricsSender = new OtlpHttpMetricsSender(new OkHttpSender(), config);
+        OtlpMeterRegistry meterRegistry = OtlpMeterRegistry.builder(config).metricsSender(httpMetricsSender).build();
+        // end::customizeHttpSender[]
+    }
+
+    @Test
+    void customizeOtlpSender() {
+        // tag::customGrpcSender[]
+        OtlpConfig config = OtlpConfig.DEFAULT;
+        OtlpMetricsSender metricsSender = new OtlpGrpcMetricsSender();
+        OtlpMeterRegistry meterRegistry = OtlpMeterRegistry.builder(config).metricsSender(metricsSender).build();
+        // end::customGrpcSender[]
+    }
+
+    @NonNullApi
+    private static class OtlpGrpcMetricsSender implements OtlpMetricsSender {
+
+        @Override
+        public void send(byte[] metricsData, Map<String, String> headers) {
+        }
+
+    }
+
+}

implementations/micrometer-registry-otlp/src/main/java/io/micrometer/registry/otlp/MetricsSender.java

@@ -0,0 +1,32 @@
+/*
+ * Copyright 2025 VMware, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.micrometer.registry.otlp;
+
+import java.util.Map;
+
+// intentionally not public while we incubate this concept
+// if we want to use this in other registries, it should move to micrometer-core and become public API
+interface MetricsSender {
+
+    /**
+     * Send encoded metrics data from a {@link io.micrometer.core.instrument.MeterRegistry
+     * MeterRegistry}.
+     * @param metricsData encoded batch of metrics
+     * @param headers metadata to send as headers with the metrics data
+     */
+    void send(byte[] metricsData, Map<String, String> headers);
+
+}

implementations/micrometer-registry-otlp/src/main/java/io/micrometer/registry/otlp/OtlpHttpMetricsSender.java

@@ -0,0 +1,83 @@
+/*
+ * Copyright 2025 VMware, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.micrometer.registry.otlp;
+
+import io.micrometer.common.util.internal.logging.InternalLogger;
+import io.micrometer.common.util.internal.logging.InternalLoggerFactory;
+import io.micrometer.core.ipc.http.HttpSender;
+
+import java.util.Map;
+
+/**
+ * An implementation of {@link OtlpMetricsSender} that uses an {@link HttpSender}.
+ *
+ * @since 1.15.0
+ */
+public class OtlpHttpMetricsSender implements OtlpMetricsSender {
+
+    private static final InternalLogger logger = InternalLoggerFactory.getInstance(OtlpHttpMetricsSender.class);
+
+    private final HttpSender httpSender;
+
+    private final OtlpConfig config;
+
+    private final String userAgentHeader;
+
+    public OtlpHttpMetricsSender(HttpSender httpSender, OtlpConfig config) {
+        this.httpSender = httpSender;
+        this.config = config;
+        this.userAgentHeader = getUserAgentHeader();
+    }
+
+    @Override
+    public void send(byte[] metricsData, Map<String, String> headers) {
+        HttpSender.Request.Builder httpRequest = this.httpSender.post(config.url())
+            .withHeader("User-Agent", userAgentHeader)
+            .withContent("application/x-protobuf", metricsData);
+        headers.forEach(httpRequest::withHeader);
+        try {
+            HttpSender.Response response = httpRequest.send();
+            if (!response.isSuccessful()) {
+                logger.warn(
+                        "Failed to publish metrics (context: {}). Server responded with HTTP status code {} and body {}",
+                        getConfigurationContext(), response.code(), response.body());
+            }
+        }
+        catch (Throwable e) {
+            logger.warn("Failed to publish metrics (context: {}) ", getConfigurationContext(), e);
+        }
+    }
+
+    private String getUserAgentHeader() {
+        String userAgent = "Micrometer-OTLP-Exporter-Java";
+        String version = getClass().getPackage().getImplementationVersion();
+        if (version != null) {
+            userAgent += "/" + version;
+        }
+        return userAgent;
+    }
+
+    /**
+     * Get the configuration context.
+     * @return A message containing enough information for the log reader to figure out
+     * what configuration details may have contributed to the failure.
+     */
+    private String getConfigurationContext() {
+        // While other values may contribute to failures, these two are most common
+        return "url=" + config.url() + ", resource-attributes=" + config.resourceAttributes();
+    }
+
+}